This invention relates to a moisture content meter utilizing the microwave absorption phenomenon of water in the sample.
In case of storing cereals, lumber, fabrics, paper, processed foods or the like for a long period, some kind of drying operation to keep the moisture contents in those materials to a certain level suitable for storage is essential for the purpose of preventing deterioration by mold or dampness or of regulating them to processing or storing conditions.
There are two commonplace ways of drying namely: (1) by the solar heat; (2) by a dryer machine. Along with the recent development, however, of farming mechanization such as of cultivators, combines, planting machines, etc., the use of dryers has been making a rapid diffusion in the cereal drying, too.
There are also two principal ways of measuring the moisture contents in cereals in the drying process, namely: (1) firstly measuring the weight of sampled cereals, then drying them by applying infrared rays, and lastly comparing the weights thereof before and after the drying; (2) firstly crushing sampled cereal, then placing it between a pair of electrodes to find the electric resistance value thereof and finding the moisture value with a prepared standard curve. However, since the first method of applying the infrared rays is too expensive, the second method of reckoning from the resistance values has generally been utilized by ordinary farmers. In either case, nevertheless, the measuring largely depends on manual operations while the cereal drying is usually day-and-night operations, thereby requiring day-and-night vigil measuring. Such standard manual operations are apt to cause over-drying or under-drying and resultant deterioration of the quality of cereals. In order to alleviate such strains in the operation and to prevent the risks of over-drying as well as under-drying, it has become an objective to develop an automatic measuring apparatus of moisture contents to be used for drying machines.
There has also been a strong demand for a means to measure moisture contents in flowing or travelling states of some kinds of cereals.
For realizing such demand as mentioned above, utilization of electromagnetic waves, especially microwaves, is effective. This formula is a utilization of the characteristic of microwaves to attenuate when passing through a substance and is based on the principle that the attenuation of microwaves has a certain relation with the moisture amount in the substance.
A typical construction of a microwave-type moisture content meter of prior art is that a pair of horn antennae are connected to a separate microwave oscillator and microwave detector, respectively, are arranged facing each other, and between which a sample object for measuring is placed or moved, its moisture content being measured by the attenuation of microwaves passing through the sample object. In such prior art apparatus, however, for a large amount of, or for bulky sample objects it is impossible to make a direct measurement on the whole object because of heavy attenuation of the microwave. On the other hand, if a measuring of part of the object is attempted, it is necessary to arrange either the horn antenna with the oscillator or the one with the microwave detector in a position buried in the flow of the flowing sample object and hence, not only does the construction of the device become complicated, but also the free flow or transit of the sample object is impeded, thereby reducing the practical value of the apparatus.
Another known method is to measure the moisture contents from the transmission loss of microwaves when the sample object contacts a microwave-guide path. In the case of employing a wave-guide path of such construction, however, it is difficult to control characteristic impedances of the wave-guide path to a target value, and therefore, it is also difficult to form a highly efficient microwave circuit with small reflection by coupling other microwave circuit parts thereto. Therefore, this method has also has a shortcoming that a practical moisture sensor is difficult to achieve.